First thing:
The increase will be in effect from September 1st on. Not before.
Second thing:
Your cancellation needs to be written in German and sent to the address that is given in the letter you received and/or named on the linked page.
Last thing:
Please keep in mind, that your cancellation will be in effect on September 1st. So you definitely WILL be offline on that date. The only exception would be if you'd make a new contract and ask your new provider to handle the cancellation process for you. They may also ask for the cancellation of the old contract on September 1st.
If you DON'T follow that procedure (e.g. ask your new ISP for the number transfer [Rufnummernportierung] and let him cancel the contract), you will definitely be offline for some days, maybe even weeks until the new line is set up correctly. That's because the new provider won't be able to use the exact same line in that case - he'll have to ask Telekom to provide him with a line and a DSL port - and that may change depending on general availability of ports.
